﻿namespace Athena.Infrastructure;

/// <summary>
/// 
/// </summary>
public static class GuidUtil
{
    private static readonly long EpochMilliseconds = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc).Ticks / 10000L;

    /// <summary>
    /// Creates a sequential GUID according to SQL Server's ordering rules.
    /// </summary>
    public static Guid NewSequentialId()
    {
        // This code was not reviewed to guarantee uniqueness under most conditions, nor completely optimize for avoiding
        // page splits in SQL Server when doing inserts from multiple hosts, so do not re-use in production systems.
        var guidBytes = Guid.NewGuid().ToByteArray();

        // get the milliseconds since Jan 1 1970
        byte[] sequential = BitConverter.GetBytes((DateTime.Now.Ticks / 10000L) - EpochMilliseconds);

        // discard the 2 most significant bytes, as we only care about the milliseconds increasing, but the highest ones 
        // should be 0 for several thousand years to come (non-issue).
        if (BitConverter.IsLittleEndian)
        {
            guidBytes[10] = sequential[5];
            guidBytes[11] = sequential[4];
            guidBytes[12] = sequential[3];
            guidBytes[13] = sequential[2];
            guidBytes[14] = sequential[1];
            guidBytes[15] = sequential[0];
        }
        else
        {
            Buffer.BlockCopy(sequential, 2, guidBytes, 10, 6);
        }

        return new Guid(guidBytes);
    }
}